1. Field of the Invention
The present invention relates to a method for purifying an exhaust gas using a transition metal-containing zeolite having a high hydrothermal stability which can be used as a catalyst and an adsorbent. More specifically it relates to a transition metal-containing zeolite, the crystal of which does not collapse even when the zeolite is used in an atmosphere containing moisture at high temperature, and further, relates to a method of producing the zeolite. The present invention further relates to a catalyst for treating an exhaust gas discharged from, for example, a boiler or an automobile engine, and containing nitrogen oxides, and to a method of using same, and more particularly, to a catalyst, for purifying an exhaust gas, having an extremely high durability, and to a method of using the catalyst.
2. Description of the Related Art
Transition metal-containing zeolites are now widely used as a catalyst in the fields of oil refining, petrochemistry and environmental purification, and as an adsorbent of a gas, or the like. In these applications, the environment in which the zeolite is used very often reaches a high temperature, and thus the development of a transition metal-containing zeolite having a high heat resistance and a high hydrothermal stability is urgently required.
It is known that the heat resistance of the transition metal-containing zeolite is affected by the kind of zeolites, and a molar ratio of SiO.sub.2 /Al.sub.2 O.sub.3 and the like, and zeolites having a higher SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio exhibit a particularly high heat resistance. Nevertheless, when the transition metal-containing zeolite is used as a catalyst or an adsorbent, and the like, the zeolite must contain a greater amount of transition metals as an active site, through an ion-exchange, and thus zeolites having a lower SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio are often required.
As mentioned above, the transition metal-containing zeolite has the problems of a low heat resistance and hydrothermal stability, and thus is not particularly useful in an atmosphere containing moisture at high temperature.
On the other hand, a selective catalytic reduction method using ammonia in the presence of a catalyst, and a non-selective catalytic reduction method which passes an exhaust gas through a catalyst and reduces nitrogen oxides by unburnt or remaining carbon monoxide and hydrocarbons, are in practical use as a method of removing nitrogen oxides in an exhaust gas discharged from, for example, a boiler or an automobile engine.
Japanese Unexamined Patent Publication (Kokai) No. 60-125250 proposes a zeolite, which is subjected to copper ion exchange, as a catalyst which can directly catalytically decompose the nitrogen oxides in the absence of a reducing agent.
Furthermore, a catalyst containing a base metal or metals, in the zeolite composition thereof has been proposed as a catalyst which can selectively reduce nitrogen oxides by reducing agents such as unburnt carbon monoxide and hydrocarbons even in the presence of an excess oxygen, to purify the exhaust gas of a Diesel engine and a lean burn engine aimed at reducing fuel consumption (see Japanese Unexamined Patent Publication (Kokai) No. 63-100919).
Nevertheless, the catalysts proposed in these prior art references still have a problem of durability, particularly at high temperatures, and thus are not in practical use.